Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. I. Toluene

The Future of Neurotoxicology: A Neuroelectrophysiological Viewpoint

Neuroelectrophysiology is an old science, dating to the 18th century when electrical activity in nerves was discovered. Such discoveries have led to a variety of neurophysiological techniques, ranging from basic neuroscience to clinical applications. These clinical applications allow assessment of complex neurological functions such as (but not limited to) sensory perception (vision, hearing, somatosensory function), and muscle function. The ability to use similar techniques in both humans and animal models increases the ability to perform mechanistic research to investigate neurological problems. Good animal to human homology of many neurophysiological systems facilitates interpretation of data to provide cause-effect linkages to epidemiological findings. Mechanistic cellular research to screen for toxicity often includes gaps between cellular and whole animal/person neurophysiological changes, preventing understanding of the complete function of the nervous system. Building Adverse Outcome Pathways (AOPs) will allow us to begin to identify brain regions, timelines, neurotransmitters, etc. that may be Key Events (KE) in the Adverse Outcomes (AO). This requires an integrated strategy, from in vitro to in vivo (and hypothesis generation, testing, revision). Scientists need to determine intermediate levels of nervous system organization that are related to an AO and work both upstream and downstream using mechanistic approaches. Possibly more than any other organ, the brain will require networks of pathways/AOPs to allow sufficient predictive accuracy. Advancements in neurobiological techniques should be incorporated into these AOP-base neurotoxicological assessments, including interactions between many regions of the brain simultaneously. Coupled with advancements in optogenetic manipulation, complex functions of the nervous system (such as acquisition, attention, sensory perception, etc.) can be examined in real time. The integration of neurophysiological changes with changes in gene/protein expression can begin to provide the mechanistic underpinnings for biological changes. Establishment of linkages between changes in cellular physiology and those at the level of the AO will allow construction of biological pathways (AOPs) and allow development of higher throughput assays to test for changes to critical physiological circuits. To allow mechanistic/predictive toxicology of the nervous system to be protective of human populations, neuroelectrophysiology has a critical role in our future.

A weight of evidence approach for the assessment of the ototoxic potential of industrial chemicals

There is accumulating epidemiological evidence that exposure to some solvents, metals, asphyxiants and other substances in humans is associated with an increased risk of acquiring hearing loss. Furthermore, simultaneous and successive exposure to certain chemicals along with noise can increase the susceptibility to noise-induced hearing loss. There are no regulations that require hearing monitoring of workers who are employed at locations in which occupational exposure to potentially ototoxic chemicals occurs in the absence of noise exposure. This project was undertaken to develop a toxicological database allowing the identification of possible ototoxic substances present in the work environment alone or in combination with noise exposure. Critical toxicological data were compiled for chemical substances included in the Quebec occupational health regulation. The data were evaluated only for noise exposure levels that can be encountered in the workplace and for realistic exposure concentrations up to the short-term exposure limit or ceiling value (CV) or 5 times the 8-h time-weighted average occupational exposure limit (TWA OEL) for human data and up to 100 times the 8-h TWA OEL or CV for animal studies. In total, 224 studies (in 150 articles of which 44 evaluated the combined exposure to noise and a chemical) covering 29 substances were evaluated using a weight of evidence approach. For the majority of cases where potential ototoxicity was previously proposed, there is a paucity of toxicological data in the primary literature. Human and animal studies indicate that lead, styrene, toluene and trichloroethylene are ototoxic and ethyl benzene, n-hexane and p-xylene are possibly ototoxic at concentrations that are relevant to the occupational setting. Carbon monoxide appears to exacerbate noise-induced hearing dysfunction. Toluene interacts with noise to induce more severe hearing losses than the noise alone.

Occupational ototoxicity of n-hexane

The ability of chemicals to produce hearing loss themselves or to promote noise-induced hearing loss has been reported for some organic solvents. The objective of this study was to review the literature on the effects of low-level exposure to n-hexane on the auditory system and consider its relevance for occupational settings. Both human and animal investigations were evaluated only for realistic exposure concentrations based on the permissible inhalation exposure limits. In Quebec, the time-weighted average exposure value (TWAEV) for 8 h is 50 ppm. In humans, the upper limit for considering ototoxicity data relevant to the occupational exposure situation was set at five times the TWAEV. Animal data were evaluated only for exposure concentrations up to 100 times the TWAEV. There is no convincing evidence of n-hexane-induced hearing loss in workers. In rats, n-hexane seems to affect auditory function; however, the site of these alterations cannot be determined from the present data. Further studies with sufficient data on the exposure of workers to n-hexane are necessary to make a definitive conclusion. In the interim, we recommend considering n-hexane as a possibly ototoxic agent.

Effects of Concurrent Noise and Jet Fuel Exposure on Hearing Loss

We sought to examine the effects of occupational exposure to jet fuel on hearing in military workers.

METHODS

Noise-exposed subjects, with or without jet fuel exposure, underwent hearing tests. Work histories, recreational exposures, protective equipment, medical histories, alcohol, smoking, and demographics were collected by questionnaire. Jet fuel, solvent, and noise exposure data were collected from records. Fuel exposure estimates were less than 34% of the OSHA Threshold Limit Values.

RESULTS

Subjects with 3 years of jet fuel exposure had a 70% increase in adjusted odds of hearing loss (OR = 1.7; 95% CI = 1.14-2.53) and the odds increased to 2.41 (95% CI = 1.04-5.57) for 12 years of noise and fuel exposure.

CONCLUSIONS

These findings suggest that jet fuel has a toxic affect on the auditory system.

Occupational exposure to chemicals and sensory organs: a neglected research field

The effect of industrial chemicals on the sensory perception of exposed workers has received scant attention from the medical community to date, and the scientific literature is mainly limited to some case-reports or isolated studies. Possible explanations for this include the complexity of sensory perception, and the lack of agreement among researchers on methods for testing large groups of subjects. Nevertheless, some published studies showed that vision, hearing and olfactory function can be affected by various industrial metals and solvents, and some data exist also for touch and taste. This review discusses the main industrial chemicals involved. The pathogenesis of the toxicity of chemicals to sensory perception may be related to an action on receptors, nerve fibers, and/or the brain; probably, different pathogenetic mechanisms are involved. One of the main problems in this research field is that most of the studies to date evaluated the effect of a single industrial chemical on a single sense: as an example, we know that styrene exposure can impair smell and also hearing and vision but we have little idea whether different senses are impaired in the same worker, or whether each impairment is independent. In addition, workers are frequently exposed to different chemicals: co-exposure may have no effect, or result in both an increase or a decrease of the effect, as was observed for hearing loss, but studies on this aspect are largely insufficient. Research shows that both occupational and environmental exposure to industrial chemicals can affect sense organs, and suggests that the decline of perception with age may be, at least partly, related to this exposure. Nevertheless, available evidence is incomplete, and is largely inadequate for an estimation of a "safe" threshold of exposure. Good quality further research in this field is needed. This is certainly complex and demands adequate resources, but is justified by the ultimate result: the possibility to prevent an avoidable part of the decline in sensory function with age.

The ototoxic interaction of styrene and noise

The interaction between noise and inhaled styrene on the structure and function of the auditory organ of the male Wistar rat was studied. The animals were exposed either to 600 ppm, 300 ppm or 100 ppm styrene (12 h/day, 5 days/week, for 4 weeks) alone or in combination with a simultaneous 100-105 dB industrial noise stimulant. Auditory sensitivity was tested by auditory brainstem audiometry at 1.0, 2.0, 4.0 and 8.0 kHz frequencies. Inner ear changes were studied by light microscopy. Exposure to 600 ppm styrene alone caused a 3 dB hearing loss only at the highest test frequency (8 kHz). Quantitative morphological analysis of cochlear hair cells (cytocochleograms) showed a severe outer hair cell (OHC) loss particularly in the third OHC row of the upper basal and lower middle coil. Exposure to noise alone caused only a mild hearing loss (2-9 dB), and only an occasional loss of OHCs (<1% missing). Exposure to the combination of noise and 600 ppm styrene caused a moderate flat hearing loss of 23-27 dB. The cytocochleograms showed a more severe damage of the OHCs than after exposure to 600 ppm styrene alone. The inner hair cells were found to be destroyed in some animals in the upper basal turn only after the combination exposure. Only in combination with noise exposure, the lower styrene concentrations (100 and 300 ppm) induced a hearing loss which was equivalent to that seen after exposure to noise alone. We conclude that: (1) There is an ototoxic interaction between styrene and noise. (2) Synergism is manifested only if styrene is applied in concentrations above the critical level (between 300 and 600 ppm in this study).

A system for simultaneous multiple subject, multiple stimulus modality, and multiple channel collection and analysis of sensory evoked potentials

A system has been developed for collecting sensory evoked potentials simultaneously from multiple channels for multiple subjects at up to 80 kHz sample rate per channel. Sample rates up to 200 kHz are available for four or less chambers and a single channel per chamber. A variety of visual, somatosensory, and auditory stimuli may be presented singly or simultaneously. Collected waveforms are associated with searchable text (metadata) to allow convenient selection from a relational database. Multiple waveforms can then be easily grouped for analysis and processed. Results can be exported to other software for further graphics or statistical processing. Scripting and event logging are available to provide automation and improve data confidence. Sample data are presented from control animals for each of the sensory modalities for comparison with historical data collected from other systems.

Effect of inhalation exposure to 2-bromopropane on the nervous system in rats

Exposure to 2-bromopropane (2-BP) is suspected to have adverse effects on the nervous system. The aim of this study was to investigate whether the exposure of rats to 2-BP had neurotoxic effects using histological and electrophysiological studies. Wistar strain male rats were exposed daily to either 100 or 1000 ppm 2-BP or to fresh air for 8 h a day for 12 weeks. Body weight was measured before exposure and every 2 weeks. Motor nerve conduction velocity (MCV) and distal latency (DL) were measured before exposure and every 4 weeks during exposure. Histological examination of the nervous system was also performed. Exposure of rats (n = 9) to 1000 ppm resulted in suppression of body weight gain and a significant decrease in brain weight compared to the control (n = 9). Electrophysiological measurements showed a significant decrease in MCV in 1000 ppm exposed rats at 8 weeks and significant prolongation of DL at 8 and 12 weeks. Abnormalities of the myelin sheath were detected in the common peroneal nerves. In 100-ppm exposed rats (n = 9), no significant changes were noted in body weight and the peripheral nerve. In conclusions, long-term exposure to 1000 ppm of 2-BP may result in peripheral neuropathy in rats.

Hearing loss from combined exposures among petroleum refinery workers

Workers from a refinery (n = 438) were interviewed, had their hearing tested and had their exposures to noise and solvents assessed. Measurements suggested that most exposures to noise and solvents were within exposure limits recommended by international agencies; however, the prevalence for hearing loss within the exposed groups ranged from 42 to 50%, significantly exceeding the 15-30% prevalence observed for unexposed groups. The adjusted odds ratio estimates for hearing loss were 2.4 times greater for groups from aromatics and paraffins (95% CI 1.0-5.7), 3 times greater for the maintenance group (95% CI 1.3-6.9) and 1.8 times greater for the group from shipping (95% CI 0.6-4.9), when compared to unexposed workers from the warehouse and health clinic. The results of acoustic reflex decay tests suggest a retrocochlear or central auditory pathway involvement in the losses observed in certain job categories. These findings indicate that factors in addition to noise ought to be considered when investigating and preventing occupational hearing loss.

Differing non-additive alterations in different parts of the nervous system of the rat

Synergistic loss of auditory sensitivity has been observed after combined exposure to xylene and n-hexane, and to toluene and n-hexane. After these combined exposures, antagonisms were seen in the auditory pathway, visual pathway, peripheral nerve and testes. synergistic interaction in one part of the nervous system and antagonistic interaction in another part has also been seen after combined exposure to toluene and dichloromethane. Two conclusions can be drawn from these observations: (1) an antagonism seen in one part of the nervous system after exposure to a specific combination of chemicals does not exclude the possibility of a synergism in other parts; and (2) both synergism and antagonism can occur within the same neurosensory organ after identical combined exposure. The observed, presumably cochlear, loss of auditory sensitivity after exposure to xylene and n-hexane was synergistic, while antagonism was found in the auditory pathway. These observations suggest that extrapolation of the combined effect in one part of the nervous system to another part, even within the same organ system, can be unreliable.

Auditory and vestibular functions after single or combined exposure to toluene: a review

Toluene is a widely used organic solvent, heavily employed in many manufacturing industries. Recently, evidence has begun to accumulate on the deleterious effect of toluene exposure has on the auditory and vestibular systems. Although little published information exists regarding these effects, the reported findings indicate a need for further investigation. The results of such investigations may dramatically affect occupational hearing conservation practices and legislation. Both human and animal studies will be summarized in discussing the effects of toluene alone or in combination with noise or other chemicals. Gaps in scientific knowledge are highlighted to assist future research.

Function of the auditory system, the visual system, and peripheral nerve and long-term combined exposure to toluene and ethanol in rats

Male pigmented rats (n = 36) were exposed to toluene and/or ethanol (1000 p.p.m. toluene in the inhaled air 21 hr/day, and 5.7-8.0% ethanol in the drinking water continuously) during 8 weeks. Electrophysiological recordings were made 1 week after the exposure. Auditory sensitivity (auditory brainstem response) was reduced only after exposures including toluene. At 20 kHz, ethanol antagonized toluene-induced loss of auditory sensitivity (P < 0.05). Flash evoked potentials were not affected in any group. In peripheral nerve, exposures containing ethanol were followed by increased amplitudes of nerve and muscle action potentials. Exposures including toluene were followed by an increase in liquid consumption.

Four weeks inhalation exposure to n-heptane causes loss of auditory sensitivity in rats

The effects of exposure to 800 or 4000 p.p.m. of n-heptane, CAS No. [142-82-5]) 6 hr per day during a period of 28 days, on the function of the auditory system were examined by measurements of auditory brain stem response (ABR) in Long Evans rats. The ABR was measured simultaneously with both needle electrodes and implanted electrodes. The wave forms recorded with the two types of electrodes were similar, but the amplitudes were largest on the recordings with implanted electrodes. The overall ratio between the amplitudes obtained with implanted electrodes and with needle electrodes was 1.4 for peak Ia and 2.5 for peak IV of the ABR. The exposure to n-heptane (4000 p.p.m.) reduced the amplitudes of components Ia and IV of the ABR. The reduction was most consistent for component IV and most pronounced at higher frequencies and intensities. The reduction in ABR corresponds to an increase in the auditory threshold of approximately 10 dB at all frequencies. Neither the latencies nor the interpeak latencies of components Ia and IV were changed. No significant changes in ABR were observed in the group exposed to 800 p.p.m. The mechanism behind the ototoxicity of organic solvents is discussed.

Function of the auditory and visual systems, and of peripheral nerve, in rats after long-term combined exposure to n-hexane and methylated benzene derivatives. II. Xylene

Rats were exposed to xylene, to n-hexane, or to xylene together with n-hexane, each solvent 1000 p.p.m. (1000 + 1000 p.p.m. in mixed exposure), 18 hr/day, 7 days/week during 61 days. Neurophysiological recordings were made 2 days, 4 months, and 10 months after the end of exposure. Exposure to n-hexane alone, or xylene alone, caused a slight loss of auditory sensitivity as recorded by auditory brainstem response 2 days after the exposure. Exposure to n-hexane together with xylene caused persistent loss of auditory sensitivity (7-17 dB; P < 0.05) which was non-additively enhanced (P < 0.01). The latencies of the flash evoked potentials in the group exposed to n-hexane alone were prolonged (re C group) 2 days after exposure, while smaller prolongations were found in the group exposed to xylene together with n-hexane. Exposure to n-hexane alone caused a marked decrease in nerve conduction velocity, while simultaneous exposure to xylene inhibited n-hexane-induced velocity reduction in peripheral nerve (P < 0.01).